1. Field of the Invention
The present invention relates to a charge pumping circuit, and more particularly to a differential charge pump and method therefor, and a phase locked loop circuit and method therefor using the pump and method for generating a differential output signal in response to a differential input signal.
2. Description of the Related Art
Generally, a phase locked loop includes a phase detector, a charge pump, a loop filter, and a voltage controlled oscillator. The phase detector detects a phase difference between a reference clock signal and a clock signal supplied from the voltage controlled oscillator. The charge pump charges or discharges the loop filter in response to an output signal of the phase detector. The voltage controlled oscillator varies a phase of the clock signal output therefrom in response to a voltage corresponding to the charge in the loop filter.
The charge pump of the phase locked loop is classified as one of two types: a single charge pumpor a differential charge pump. The single charge pump generates a single output signal, while the differential charge pump generates a differential output signal. The voltage controlled oscillator of the phase locked loop using the single charge pump generates clock signals in response to the single output signal, and the voltage controlled oscillator of the phase locked loop using the differential charge pump generates clock signals in response to the differential output signals.
Generally, in the case of a phase locked loop employing the single charge pump, switching noise is generated by performing a switching operation in response to up and down signals output from the phase detector. It is not easy to remove the noise applied to the next stage, a loop filter, since the phase locked loop generates a single output signal.
In addition, when a phase locked loop is designed by employing the differential charge pump, the switching noise is also generated by performing the switching operation in response to the up and down signals output from the phase detector. However, since the phase locked loop generates a differential output signal, the effect caused by the noise can be removed even though the noiseis applied to the next stage, the loop filter.
For the reason described above, the phase locked loop is generally designed by employing the differential charge pump rather than the single charge pump. However, even though the phase locked loop is designed using the differential charge pump, there is still a drop in performance by the switching noise occurring when the differential charge pump performs the switching operation.
Therefore, studies to reduce the switching noise of the phase locked loop employing the differential charge pump have been continued, and many technologies have been disclosed
FIG. 1 shows the construction of an example of a conventional differential charge pump, which is disclosed in FIG. 4 of U.S. Pat. No. 6,385,265.
Since the construction and the operation of the differential charge pump is shown in FIG. 1 and disclosed in detail in the patent publication, only a brief explanation will be given in this specification.
The differential charge pump shown in FIG. 1 is constructed wherein up signals PUMPUPN, PUMPUPP and down signals PUMPDNN, PUMPDNP are applied to the gates of PMOS transistors 170a, 170b, 180a and 180b and NMOS transistors 172a, 172b, 178a and 178b and buffers 200a and 200b are connected between a first output signal FILTU generating terminal and a second output signal FILTD generating terminal. The switching noise generated on the switching operation of PMOS transistors 170a, 170b, 180a and 180b and NMOS transistors 172a, 172b, 178a and 178b, by up signals PUMPUPN, PUMPUPP and down signals PUMPDNN, PUMPDNP, can be removed by the buffers 200a and 200b. 
The above construction has a disadvantage that it is difficult to remove the switching noise completely using the buffer 200a and 200b. Another disadvantage of the above construction is that the circuit structure is too complicated.